In recent years, advancement made in the development of high strength and fast cure structural adhesives has broadened their application into the area of automotive assembly processes. In the beginning, structural adhesives were used to bond automotive components molded of fiberglass reinforced plastics. For instance, the rear door of some of the full size wagons manufactured by General Motors Corp. is constructed of large pieces molded of sheet molding compound adhesively bonded together by structural adhesives. The Corvette model manufactured by the Chevrolet Division also has molded pieces of fiberglass reinforced plastics bonded together by structural adhesives.
Adhesives have been used in bonding steel parts together in other automobile structures such as in the roof, the door, and other parts of a vehicle. However, the adhesives used in these applications are not of structural strength and are used primarily for sealing purposes to prevent moisture penetration. Problems encountered when attempting to bond load bearing steel components together at sufficient structural strength in an automobile assembly environment are numerous. First, the adhesive must cure to a minimum strength in a reasonably short period of time compatible with automotive assembly operations. Secondly, the adhesive bond must achieve a minimum bond strength sufficient for the assembly to endure normal operating conditions of a vehicle, for instance, to sustain various load and temperature extremes. Thirdly, the adhesive bond must retain its strength after extended high temperature exposure frequently seen in paint bake cycles.
While some conventional 2-part structural adhesives are adequate in bonding non-treated steel surfaces together, their effectiveness is greatly reduced when the steel surfaces are pretreated. Today, virtually all steel components in a vehicle are treated for corrosion protection and for paint process preparations. One of such treatments widely used is the electrodeposition of an organic primer on a phosphated steel surface. In this process, steel surfaces are first phosphated by growing a thin layer of zinc phosphate crystals on ferric oxide, that is, the oxidized surface of bulk steel. An acrylic based primer layer is then deposited on top of the zinc phosphate. The thickness of this layer depends on the particular process but typically is in the range of from about 1 to 10 microns.
One of the most widely used structural adhesives is of the epoxy type. When a conventional 2-part epoxy formulation typically having a mix ratio of 1:1 is used on steel surfaces electrodeposited with an organic primer, very low initial strength is obtained. The strength loss after water soak and salt spray environments are also extensive. These undesirable results were traced to chemical and physical degradation of the organic primer used in the electrodeposition process. Visual inspection of the failure surfaces revealed a mixed failure mode in the phosphate/primer/adhesive interface region. In addition, certain surfaces exhibited extensive blistering and degradation of the organic primer layer. There is sufficient indication that the organic primer was attacked and degraded by the epoxy adhesive.